Some objects do not return to their original shape after the removal of force. these object are called inelastic objects.
What is the definition of perfectly inelastic collision?Unlike an elastic collision, in which the objects stick together by conserving both momentum and kinetic energy, an inelastic collision conserves momentum, but it loses the kinetic energy. During an inelastic collision, the kinetic energy transforms into heat, sound or light energy.
Swinging balls are an example of elastic collision. The ball on one end strikes the remaining balls that are hanging in a straight line stuck on each other. As soon as the ball strikes the first ball, the momentum and the kinetic energy of the collision pass through the entire line of balls. Hence, the last ball on the other end of the line is out. In a PIC, as soon as the first ball strikes the remaining balls, the momentum makes two balls to come out at the other end of the line, although the kinetic energy is lost.
Typical examples of inelastic collision are between cars, airlines, trains, etc.
For instance, when two trains collide, the kinetic energy of each train is transformed into heat, which explains why, most of the times, there is a fire after a collision. However, the momentum of the two trains that are involved in the collision remains unaffected. So, the trains collide with all their speed, maintaining their momentum, yet their kinetic energy is transformed into heat energy.
Another way to explain a train or a car collision is this: when the two trains or cars collide, they stick together while slowing down. They slow down because their kinetic energy is gradually lost. Still, they collide because they conserve their momentum.
Most of the collisions between large-scale objects are PICs.